What is the meaning of the abbreviations in the email headers and on the status screen?

These abbreviations in the X-Spam-Score headervariable and on the statusscreen, tell you which tests gave results during heurastic analysis.

These test are telling you what the reason is why a message is markes as spam or not.
Every test resembles a certain score. The sum of all the scores together is also in the same header variable.

If for example the following header variable is present in your message:

you can read it as following:

Hits
Resembles the total score of the message. In this case 6.595.

Tests
A list of the tests resulting in a score for this message. The tests meaning are here:

BAYES_80
The message does has a bayesian spam probability of 80 to 95%. Individual score: 3.608

BIZ_TLD
The message contains an URL in the BIZ top-level domain. Individual score: 2.434

MSGID_OUTLOOK_INVALID
Indicates that the message-id is fake (in Outlook Express format). Individual score: 2.498

HTML_30_40
Indicates that the message is 30% to 40% HTML-based. Individual score: 0.437

HTML_MESSAGE
Indicates that HTML is included in the message. Individual score: 0.001

HTML_TAG_EXIST_TBODY
Indicates that the message has a TBODY tag included. Individual score: 0.079

Summed together these individuls results are 6.559, which will obviously result as marked as spam. For a complete list of tests performed look at the Spamassassin: Tests Performed page.